(1) Field of the Invention
This invention relates generally to color space transformation of image sensors and relates more particularly to a transformation of analog signals from red, green, blue and white image sensors to values of another color space as e.g. YCbCr color space by a transformation in an analog domain.
(2) Description of the Prior Art
Color is the perceptual result of light in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm, incident upon the retina. The spectral distribution of light relevant to the human eye is often expressed in 31 components each representing a 10 nm band.
The human retina has three types of color photoreceptors cone cells, which respond to incident radiation with somewhat different spectral response curves. Because there are exactly three types of color photoreceptor, three numerical components are necessary and sufficient to describe a color, providing that appropriate spectral weighting functions are used. These cones provide the “photopic” vision.
Photoreceptors are not distributed evenly throughout the retina. Most cones lie in the fovea, whereas peripheral vision is dominated by rods. Rods handle the short wavelength light, up to about 510 nm. The number of rods is much higher than the number of cones. They are very sensitive to very low levels of light. These rods provide the “scotopic” vision. There is no color in scotopic vision and it is processed as grayscale.
Since the human eye has more photoreceptors handling black and white compared to colors luminance is more important to vision as colors.
Pixel values in accurate gray-scale images are based upon broadband brightness values. Pixel values in accurate color images are based upon tristimulus values. Color images are sensed and reproduced based upon tristimulus values, whose spectral composition is carefully chosen according to the principles of color science. As their name implies, tristimulus values come in sets of three. In most imaging systems, tristimulus values are subjected to a non-linear transfer function that mimics the lightness response of vision. Most imaging systems use RGB values whose spectral characteristics do not exactly match the tristimulus values of the human eyes.
A combination of real world physical characteristics determines what the human vision system perceives as color. A color space is a mathematical representation of these characteristics. Color spaces are often three-dimensional. There are many possible color space definitions.
Digital cameras have either RGB representation (RGB in one pixel), Bayer representation, wherein the pixels are arranged in a 2.times.2 cell having one red (R) pixel, one blue (B) pixel and two green (G) pixels, or having additionally white pixels as described in U.S. Ser. No. 10/859,797 filed: Jun. 3, 2004.
Usually different color spaces are being used to describe color images. YUV and YcbCr color spaces are getting more and more important.
The YUV color space is characterized by the luminance (brightness), “Y”, being retained separately from the chrominance (color). There is a simple mathematical transformation from RGB: Y is approximately 30% Red, 60% Green, and 10% Blue, the same as the definition of white above. U and V are computed by removing the “brightness” factor from the colors. By definition, U=Blue−Yellow, thus U represents colors from blue (U>0) to yellow (U<0). Likewise V=Red−Yellow, thus V represents colors from magenta (V>0) to Cyan (blue green) (V<0)
The YCbCr color space was developed as part of recommendation CCIR601. YCbCr color space is closely related to the YUV space, but with the color coordinates shifted to allow all positive valued coefficients:Cb=(U/2)+0.5Cr=(V/1.6)+0.5,wherein the luminance Y is identical to the YUV representation.
FIG. 1 prior art shows a typical configuration how to transform analog RGB signals to digital YCbCr signals. The analog signals of red (R), green (G) and blue (B) are being converted from analog to digital values by analog-to-digital converters 1. In the next block 2 the digital output of the converters 1 are being processed, this means, bad pixel correction, color boost, etc is performed. In block 3 a conversion from RGB to e.g. YCbCR signals is performed using a processor or the like.
The conversion from RGB to another color space is a complex and time-critical process. It is a challenge for the designers of digital color image system to provide a fast and simple means to perform such a transformation.
There are various patents known dealing with transformation to a color space such as YCbCr.
U.S. patent (U.S. Pat. No. 6,486,889 to Meyers et al.) describes an apparatus and methods in accordance with an exemplary embodiment of the invention to convert RGB video to at least one video output of Lab video, YCbCr video, with or without gamma correction, and Srgb video, with or without gamma correction. This conversion increases the flexibility of image information communication by enabling RGB video to be provided to a device that can only handle one of Lab, Srgb and/or YCbCr video.
U.S. patent (U.S. Pat. No. 6,456,325 to Hayashi) discloses an image signal-processing device including an RGB-YC conversion for converting color component signals output from a CCD (Charge Coupled Device) image sensor to luminance signals and components thereof lying in a high frequency range. The luminance signals and their components lying in a high frequency range are fed to a first, a second and a third low pass filter (LPF), respectively. Luminance signals output from a third LPF are fed to an adder while components lying in a high frequency range are fed from the first LPF to a selector. Further, the components output from the second LPF are fed to the selector via a resolution correcting section. The selector selects either one of the two different kinds of components input thereto. The adder adds the luminance signals output from the third LPF and the components selected by the selector and thereby outputs second luminance signals. A false signal reducing section reduces false signals contained in the second luminance signals and appearing at horizontal color boundaries and feeds the resulting luminance signals to a contour correcting section. With this configuration, the device is capable of reducing false signals appearing at horizontal color boundaries.
U.S. patent (U.S. Pat. No. 5,841,422 to Shyu) describes a method and an apparatus for reducing the number of matrix operations when converting digitized RGB color space signals to digitized YCbCr color space signals, at least two color difference signals, each being in terms of any two of the digitized RGB color space signals, are generated before performing first, second, third and fourth matrix multiplication operations of the color difference signals. The first and second matrix multiplication operations have first and second results to be used in conversion for the digitized Y color space signal. The third matrix multiplication operation has a third result to be used in conversion for the digitized Cb color space signal. The fourth matrix multiplication operation has a fourth result to be used in conversion for the digitized Cr color space signal.